1. Field of the Invention
This invention relates generally to the control of motors, and more particularly, to a motor control providing current limit control as a function of the maximum allowable temperature in the switching devices.
2. Description of the Related Art
A motor control for a brushless DC motor typically produces a current command signal to an input of a pulse width modulator controlling semiconductor switching devices which switch current from a power source to the motor. The switching devices are typically transistor switches. For purposes of this disclosure, "nominal current" shall mean that value of current flow through the switching devices which, over time, produces the maximum allowable switching device temperature. For purposes of this disclosure, "peak current" shall mean the maximum instantaneous current which the switching devices are capable of delivering. Typically, peak current is an integer multiplier of, for example, twice, the nominal current. A current overload is defined by a commanded current in a range between the nominal and peak currents.
Operating the switching devices with a current overload will raise their temperature above the maximum allowable switching device temperature. When this occurs, the switching devices will fail catastrophically. The time required to reach the maximum allowable switching device temperature is a function of the thermal characteristics of the switching device and the heat sink and magnitude of the current overload.
Many applications require that the motor be commanded through a series of rapid changes of velocity which may include reversals in the direction of operation. To achieve the necessary acceleration and deceleration rates, the desired commanded current will substantially exceed more typical operating currents. Design and cost limitations generally require that the size of the transistors and their heat sinks be limited thereby establishing a nominal current that is less than the anticipated maximum current. Therefore, commanding the maximum current required by severe accelerations and decelerations will result in the switching devices being operated at a current overload. In other situations, the motor may be stalled because its load has encountered an obstacle or there is some other unforeseen occurrence inhibiting its motion. When that happens, the servo control loop around the motor will produce a large error signal which in turn will eventually cause the rated peak current to be commanded. That condition must be quickly detected and limited to avoid damage to the switching devices.
Prior motor controls have analog circuits that limit the application of current overloads to protect the transistor switches from overheating. For example, they may limit the period of time that an overload current may be conducted by the transistor switch in proportion to the magnitude of the overload. However, prior art limiting circuits are typically analog circuits that measure the current overload in terms of an Ohm's Law I.sup.2 R model. The limiting circuit produces a linear limiting function in which the slope of limiting function is changed as a function of the current overload. Therefore, the period of time the switching device is conducting a current overload is regulated to protect the device from the excessive build-up of heat. A current limit circuit using a linear limiting function has several disadvantages. First, limiting current as a linear time based function to values below the peak current generally underutilizes the capability of the switching device to operate with current overloads. Second, a limiting function changing linearly with time does not accurately respond to either rapidly changing current commands or constant current command values. Such a circuit may limit overloads too quickly yielding suboptimal performance, or not quickly enough thereby permitting the switching device to overheat. Further, it does not permit current overloads to be accurately controlled in accordance with the manufacturer's overload versus time function. Finally, such analog circuits are not easily adjusted from one motor drive to another.